Engineering technicians have many nightmare stories when it comes to on-site troubleshooting. In this instance, I was called to help troubleshoot a Closed Loop (Sensorless Vector) Variable Frequency Drive (VFD) on a production line that was so large, it seemed to go so far down the plant, it never ended.
After overcoming the intimidation associated with the sheer size of the line, it became a question of focusing on just the Variable Frequency Drive (VFD) causing the issue. The maintenance technicians on site not only had home court advantage, but were on the phone with the Original Equipment Manufacturer (OEM) technical support. Everything on the line ran great, right up to the point where the VFD on the network in question was supposed to start turning a set of rolls.
The VFD was a proprietary brand that was made in Japan with the rest of the machine. After speaking with technical support from Japan, a list of discrete inputs to the Programmable Logic Controller (PLC) on the network were determined that could be checked. The maintance guys at this site were pretty sharp so they had already checked the same list twice. It is still good practice when troubleshooting these types of problems to check these discrete inputs.
The problem most have with troubleshooting a Variable Frequency Drive (VFD) controlled by a network is that it seems so intangible. There are no wires going to the start terminal to throw a meter on. This VFD, like most others, had a keypad on its front. These circumstances brought a phrase to mind that was often published on the cover of Love temperature controllers stating, “If all else fails, please read these instructions.” The customer had a manual on the VFD, looking to the manual is always a good first step to discovering the root of the issue.
When troubleshooting communications between a VFD and a PLC on the network it is necessary to look at the VFD‘s “read only” parameters and look for the answers to these two important questions:
- Is the Variable Frequency Drive (VFD) getting a run comand?
- Is the VFD getting a speed referance?
After getting familiar with the keypad, the customer was requested to get the machine running again. With the information learned from the manual, through the keypad it was determined the VFD was getting a run command and was, in fact, going into run.
However, it was also discovered that the speed reference was running at 0%. Normally a speed reference on a VFD will run between 0% and 100%. So in this instance, it was running at 0%. So the VFD was running great at 0 speed, but 0 speed is not moving. After discussing it over the phone with the OEM technical support, it had turned out the rung within the PLC on the network turning on the speed reference was not giving the proper speed reference to the VFD. Now the customer had something to work with and were able to resolve the issue through the PLC.
Keypads can be very useful when troubleshooting Variable Frequency Drives (VFDs) controlled by networks. Sometimes it is just a question of getting familliar with the way each unique VFD handles network communications.